Split fibers from blends of polypropylene and polyethylene

ABSTRACT

SPLIT FIBERS EXHIBITING THE RETICULAR STRUCTURE OBTAINED BY MECHANICAL REDUCTION OF AN EXTRUDED AND STRETCHED POLYMER FILM STRIP, THE POLYMER CONSISTING ESSENTIALLY OF A MIXTURE OF POLYPROPYLENE HAVING MORE THAN 5% ATACTIC CONTENT WITH A LOW-PRESSURE POLYETHYLENE. THE RESULTING SPLIT FIBERS ARE GENERALLY USEFUL AS CONTINUOUS THREADS, YARNS AND THE LIKE OR AS STAPLE FIBERS.

United States Patent Oifice U.s. (1260-897 A 4 Claims ABSTRACT OF THEDISCLOSURE Spit fibers exhibiting the reticular structure obtained bymechanical reduction of an extruded and stretched polymer film strip,the polymer consisting essentially of a mixture of polypropylene havingmore than 5% atactic content with a low-pressure polyethylene. Theresulting split fibers are generally useful as continuous threads, yarnsand the like or as staple fibers.

-'The present application is a division of Application Ser. No. 787,211,filed Dec. 26, 1968, now US. Pat. 3,624,194.

The production of so-called split fibers has been known for some time,e.g. as disclosed in German Patent specification 667,237. These splitfibers are produced in a specific maner which distinguishes them fromthe more conventional spinning'of individual filaments from a polymermelt or a polymer solution. Thus, in order to obtain split fibers, afiber-forming polymer such as polyvinyl chloride, cellulose acetate orthe like is first extruded as a relatively thin film or foil. The filmis then suitably cut or broken away into narrow strips or bands andstretched several times their original length. Then, one makes use ofthe tendency of the film strips to split or crack in the longitudinaldirection, and the strips are subjected to this splitting action,preferably by applying physical pressure, bending or flexing, using anyconventional mechanical means such as brushes, twisters, abraders,rollers and the like. In general, the resulting split fibers areconnected together with a network-like or reticular structure, i.e.,complete separation of the split fibers from one another is notessential.

The resulting thread-like or filamentary products can be further workedinto split fiber yarns, ropes, threads and the like, and can be used inconventional manner to achieve specialty fabrics or other textileproducts. If desired, the split fibers can be cut or otherwise reducedto staple fibers, i.e., fibers of a short staple length, which can beused alone or worked into threads and yarns with other natural orsynthetic fibers.

The term split fibers is therefore employed herein and in theaccompanying claims to identify the particular type of fibers which areproduced from an extruded film which has been separated into narrowstrips, stretched and split or cracked by mechanical means. Such splitfibers offer certain technical advantages over the spinning of a melt orsolution through spinning nozzles having round or profiled openings toform individual continuous filaments. Thus, whereas the use of suchspinning nozzles requires careful separation of the individual spunfilaments, the production of split fibers proceeds from an initialextrusion of a film or foil, after which this extruded product isreadily reduced to split fibers. However, this technology of makingsplit fibers has found only a limited application, in part due to thefact that various polymers do not always yield satisfactory results interms of yielding good commercial products.

3,819,769 Patented June 25, 1974 For example, it is known that one canproduce split fibers from polypropylene which is recognized as avaluable fiber-forming polymer. However, in order to obtain usefulproducts, the polypropylene must be substantially free of any atacticcontent, i.e. it must be an essentially isotactic polymer, one which isstereoregulated along the molecule chain. The distinction betweenatactic and isotactic polypropylene is explained in detail in the priorart, e.g. in the book Polypropylene by Kresser, Reinhold PublishingCorp., New York (1960). For purposes of the present invention, theisotactic portion of the polymer is determined as that portion whichremains insoluble when the polymer is extracted with boiling n-heptane,the dissolved or extracted portion being the atactic polymer.

When using polypropylene with an atactic content of 5% or more, it willbe found that extruded films or film strips of the polymer exhibit onlya slight tendency to split and do not yield a satisfactory split fiberproduct. Also, the deformability of the polymer at the extrusion nozzleor slot in the production of the film is very poor. Moreover, the filmor film strips do not stretch well in attempting to achieve theessential orientation of the polymer molecules during the conventionalstretching procedure, and the polymer itself has a relatively dull andunsatisfactory appearance. For these reasons, polypropylene has not beenacceptable for use in the form of split fibers, especially where thereis any substantial amount of atactic polymer present therein.

Thus, in order to achieve a practical use of polypropylene in the formof split fibers, it must first be subjected to a special extractionprocess in order to remove the bulk of its atactic content to at least5% and preferably to a proportion of less than 5%. However, thisextraction is quite troublesome and expensive so as to substantiallyraise the cost of the split fibers.

In general, it is known that polypropylene can be mixed with very slightadditions of other polymers, e.g. polyesters, polyamides or polystyrene,so as to use such mixtures in producing split fibers as disclosed inBritish Pat. No. 1,040,663. However, such additions do not materiallychange the necessity for using an essentially isotactic polypropylene.

One object of the present invention is to provide a process andespecially a novel composition consisting predominately of polypropylenefor the production of split fibers wherein the polypropylene can retaina high atactic content so as to avoid the expensive preliminaryextraction of the atactic portion of the polymer. Another object of theinvention is to provide novel split fibers of apolypropylene/polyethylene mixture which are considerably easier toproduce and lead to highly useful fibrous products even though theyretain a high content of the atactic polymer. These and other objectsand advantages of the invention will become more apparent from thefollowing detailed disclosure.

It has now been found, in accordance with the invention, that highlyimproved results can be achieved in the conventional production ofpolypropylene-containing split fibers if the individual steps ofextruding the fiberforming polymer into a film and then reducing thefilm to split fibers is carried out with a polymer consistingessentially of a homogeneous melt blend of a major proportion ofpolypropylene having an atactic content of more than 5% and a minorproportion of a high molecular weight low-pressure polyethylene. Ingeneral, the polypropylene can contain up to as much as 40% of theatactic form of the polymer, but preferably not more than 30% and mos-tdesirably about 15 to 30% by weight (with reference to the polypropylenecomponent in the polymer mixture). Also, the polymer mixture or meltblend should ordinarily contain at least 60% by weight of polypropylene,and especially suitable mixtures contain about 5 to 25% by weight of thelow-pressure polyethylene component to about 75 to 95% by weight of thepolypropylene component.

The polypropylene is prepared according to conventional processescapable of yielding a relatively high proportion of the isotacticpolymer, it being understood that the polypropylene used in the presentinvention is an isotactic polypropylene which still contains more thandefinition of a polypropylene containing a certain atactic 5% by weightof atactic constituents. In other words, the content inherently definesthe remaining content as being that portion which is not dissolved orextracted by boiling n-heptane, i.e., the isotactic content.

The polypropylene can contain catalyst residues, e.g. titanium compoundswhich have been introduced for polymerization of the initial propylenemonomer. It will be recognized, of course, that those catalysts areemployed which favor a relatively high isotactic content in the polymerproduct even though the atactic content is generally more than 5%,usually from at least up to about 60%. Where the initial isotacticcontent is less than 70%, it is preferable to carry out a relativelysimple extraction of part of the atactic polymer so as to achieve anisotactic content of preferably more than 70% up to about 9095%. In mostcases, depending upon the stereoregulation effect of the catalyst, noextraction procedure is necessary. This represents a significantadvantage as compared to the necessity of reducing the atactic contentdown to 5% by weight or less.

Of course, it is also possible to add conventional stabilizers to thepolypropylene to protect it against the effects of heat, light or otherirradiation. It is also selfunderstood that other additives may also beincluded in the polypropylene or the polymer mixture, e.g., dyes,pigments, fillers or the like, to the extent that such additives arecompatible with the conventional production of split fibers.

The low-pressure polyethylene employed for purposes of the inventionderives its name or identification as a low-pressure polymer from thewell-known Ziegler process and similar low-pressure polymerizations ofethylene which yield especially high molecular weight products. Furtherdata concerning the production of this low-pressure polyethylene can befound in such references as HouWink/Staverman, Chemie und Technologieder Kunststotf II/I, pages 137 if, Akademische Verlagsgesellschaft Geest& Portig, Leipzig (1963). The physical properties of low-pressurepolyethylenes are well known and are incorporated herein by reference asfully as if set forth in their entirety. For example, such polyethylenesgenerally have a melt index190 C. of more than 4.0 and exhibit a densityof from 0.945 to 0.965 gm./cc.

The polypropylene and low-pressure polyethylene are preferablythoroughly admixed, e.g., before or during residence in the extruder soas to form a homogeneous melt blend within the prescribed proportions.

According to the process of the invention, split fibers can be producedfrom the mixture of polypropylene and low-pressure polyethylene in anespecially advantageous and trouble-free operation. The polymer mixtureis capable of being extruded into films or foils without exhibiting anytendency of sticking or gumming up. The deformability of the polymermixture at the extrusion point, i.e., through the extruder die orslotted nozzle, is excellent. Furthermore, the appearance and otherproperties of the product are considerably better than those which canbe achieved by using only polypropylene having more than a 5% atacticcontent, i.e., without the addition of lowpressure polyethylene.

The procedure for reducing the initially extruded film into split fibersis conventional and does not require any special measures. Thus, theinitially extruded molten polymer mixture in the form of a film is firstsolidified in water and/or by means of cooled rollers, and is' then cutinto narrow bands or strips, preferably longitudinally of the film.These bands or strips, which may have a width for example of 25 mm. to100 mm. are then subjected to stretching in any suitable device,e.g.,-paired draw rolls or the like. It is generally desirable tostretch these strips to at least about 5:7 times their original length,and preferably in a draw ratio of 6:1 to 72:1.'When'-the' polymerconsists solely of polypropylene with a high atactic content, frequentdisturbance's occur asthe film and cut strips or bands pass through thecutting and stretching process, so that undesirable twisting or .coilingof the filmaceous material often occurs. By comparison, the process ofthe invention permits these operations to be carried out in a dependableand trouble-free-manner. This is of especially great advantage where oneis normally processing a very large number of narrow film strips orbands.

The stretching is an important step, as is well known, so as to orientthe individual film strips in a longitudinal direction and to obtainsmaller strips capable of being split into a more fiber-like product.After stretching or directly in conjunction therewith, the splittinginto the so-called split fibers is carried out according to conventionalprocedures. For example, splitting can be readily achieved by any numberof processes such as, for example, twisting, brushing, blowing with agaseous medium, drawing through an opening having a slightly narrowerwidth than that of the individual film strip or band, turning aroundrollers or onto and ofi of a series of rollers, drawing over edges, andthe like. The particular technique for reducing the film to split fibersis not of importance for purposes of the present invention.

The capacity of the polymer film, i.e., the cut and stretched strips,for splitting in a longitudinal direction can be increased in a knownmanner by the addition'of certain chemical reagents, e.g., those whichform a gas under the influence of heat and/or in combination with one ofthe reaction components. These and other known variations in theproduction of split fibers can be. readily adapted and used in theprocess of the invention. Y

The split fibers and the process according to the invention are furtherillustrated by the following example wherein parts and percentages areby weight unless otherwise specified.

EXAMPLE 100 parts of a polypropylene with an atactic content of about20% (the remaining being insoluble in boiling n-heptane) and 20 parts ofa low-pressure polyethylene (density=0.96 gm./cc.) were melt blended andextruded through a 25D extruder having a 60 mm. screw and equipped witha suitable die or extrusion slot to provide a film with a thickness ofapproximately 0.18 mm. The extruder was operated under the followingconditions:

Temperature of the entry zone-230 C. Temperature in the compressionzone-340 C. Temperature in the metering zone-280 C. Pressure1 10 atm.

Output-19 kg./hr.

This extruded film was then immediately cooled and solidified and thencut into individual longitudinal strips, each of which had a width of 80mm. These steps-were then stretched longitudinally on conventional drawrolls in a draw ratio of 9.5:1, i.e., 9.5 times their original length.

Finally, the stretched strips were twisted to cause them to split andform the co-called split fibers. A similar splitting is achieved bystrongly brushing the strips or by running them over one or more edgesunder tension.

In other tests, split fibers were formed while changing only therelative proportion of atactic content of the polypropylene and/ or theproportion of low-pressure polyethylene admixed therewith. The bestresults were obtained within the preferred ranges set forth hereinabove.

In general, the split fibers obtained according to the invention areespecially distinguished by their high tensile strength, achievingvalues of about to 5.6 grams/denier. These split fibers can be easilyprocessed into yarns, threads, ropes, twines and a wide variety oftextile materials. For example, they are especially useful as bindingstrands in harvesting operations, as wrapping twines or cords, as apacking material for valves or other machine parts or as a basiccontinuous thread or yarn for all types of fabrics and other textiles.If desired, these split fibers can be cut into staple lengths byconventional apparatus and used alone or in combination with othernatural or synthetic fibers or as a filler material to provide greaterstrength. Other uses will be apparent to those skilled in this art.

The invention is hereby claimed as follows:

1. Split fibers having a reticular structure as obtained by mechanicalreduction of an extruded and stretched film strip consisting essentiallyof a homogeneous mixture of a major proportion of polypropylene havingan atactic content of more than 5% up to 40% and a minor proportion of ahigh molecular weight low-pressure polyethylene.

2. Split fibers as claimed in Claim 1 wherein the atactic content ofsaid polypropylene is about 15 to by weight.

3. Split fibers as claimed in Claim 1 wherein said mixture containsabout 5 to 25% by weight of said lowpressure polyethylene and to byweight of said polypropylene.

4. Split fibers as claimed in Claim 3 wherein the atactic content ofsaid polypropylene is about 15 to 30% by weight.

References Cited UNITED STATES PATENTS 3,112,160 11/1963 Rush 18-543,281,501 10/1966 Coats et a1. 260897 3,418,396 12/1968 Edwards et a1.260- 897 3,624,194 11/1971 Pirot 264145 MURRAY TILLMAN, Primary Examiner20 C. J. SECCURO, Assistant Examiner US. Cl X.R.

264-145, 154, DIG 47

